A liquid electrographic imaging system includes an imaging substrate onto which a developer liquid is delivered to develop a latent image. The imaging substrate may be a permanent image receptor or, alternatively, a temporary image receptor, and may take the form of a drum, belt, or sheet. A liquid electrographic imaging system may be an electrostatic system having a dielectric material as the imaging substrate, or may take the form of an electrophotographic system having a photoreceptor as the imaging substrate. In an electrostatic system that makes use of a dielectric material, the latent image can be formed by selectively charging the dielectric substrate with an electrostatic stylus. In an electrophotographic system, the photoreceptor includes a photoconductive material that is uniformly charged, for example, with a corona charging device. A latent image can be formed on the photoreceptor by selectively discharging the photoreceptor with a pattern of electromagnetic radiation.
A multi-color imaging system may include several imaging stations that form a plurality of latent images on the imaging substrate. Each of the latent images in a multi-color imaging system is representative of one of a plurality of color separation images for an original multi-color image to be reproduced. As a latent image is formed, a development station applies developer liquid to the imaging substrate to develop the latent image.
The developer liquid includes a carrier liquid and developer particles which may include charge director and a colorant, such as a dye or a pigment. In a multi-color imaging system, each of a plurality of development stations applies an appropriately colored developer liquid to the imaging substrate to form an intermediate representation of the corresponding color separation image. A drying station dries the developer liquid applied by the development station or stations, leaving a film of developer material. The transfer station then transfers the developer material from the imaging substrate to an output substrate, such as a sheet of paper, fabric, plastic, or film, to form a visible representation of the original image. In some electrostatic imaging systems, the imaging substrate may serve as the output substrate, such that transfer is not necessary.
A development station generally includes a development device such as, for example, a development roller or belt. The operation of a development roller will be described for purposes of example. The development roller is rotated by a drive mechanism and charged with a bias potential that contributes to an electric field between the roller and the imaging substrate. The rotating, charged development roller delivers developer liquid to the surface of an imaging region of the imaging substrate to develop the latent image. The development roller typically is positioned a short distance from the surface of the substrate, enabling a thin layer of developer liquid to be delivered across the resulting gap. In a multi-color imaging system, the development process is repeated with each of a plurality of development rollers applying differently colored developer liquids to the imaging substrate to develop different color separation images.
Consistency in color density in electrographic printing, whether of the electrostatic or electrophotographic type, is important for minimizing plot to plot variability. Several variables influence the color density in electrographic printing, with the formulation of liquid toner being most important.
Conventional liquid toner comprises pigmented resin particles, isoparaffinic hydrocarbon carrier liquid (such as Isopar.TM.), and charge control agent to affect electrical properties. Although pigment, resin, and charge control agent will each be present in the liquid toner, proper toning of the latent image occurs only when the toner particle is composed of all three components.
Only an effective range of toner formulation of appropriately charged, pigmented resin particles in liquid toner is considered a "viable" toner. A decrease in color density outside an acceptable range (also known as "depletion") commonly occurs when either: (a) the concentration of viable toner solids becomes too low; or (b) the conductivity of the working strength toner dispersion becomes too high.
The mechanism for weak image development caused by working strength conductivity is not well understood. For example, U.S. Pat. No. 5,278,615 (Landa) suggests that the conductivity of the toner becomes high enough that the charge on the imaging substrate is satisfied by "electrical leakage." U.S. Pat. No. 5,442,427 (Day) suggests that free floating charge control agent and charged, unpigmented resin particles compete with viable particles for charged sites on the imaging substrate, resulting in a lower image density. In addition, Day also suggests that, as the conductivity of the toner becomes higher, the viable solids become charged to a higher degree, and fewer of them are needed to satisfy the charge on the imaging substrate, resulting in lower image density.
Common replenishment schemes add toner concentrate (typically about 12-15% total solids) to the working strength toner (typically about 2-3% total solids), usually monitored by a feedback mechanism such as optical transmissivity.
Toner concentrate can be formulated with the proper relative concentrations of pigment, resin and charge control agent, but non-viable toner solids are always present in commercial toners.
During electrographic printing, viable toner solids are carried onto the imaging substrate at a much higher rate than non-viable toner solids. Therefore, non-viable toner solids, comprising free charge control agent and charged, unpigmented resin particles, eventually build up to unacceptable levels, causing depletion as described above. At this point the toner is considered unreplenishable, and the entire fluid volume of the liquid toner must be discarded.
Increasing the applied voltage during electrographic imaging can increase image density, but can compensate for depleted toner only to a limited extent. Current replenishment schemes ultimately lead to depletion of the toner by either of the two methods discussed above, and a decrease in color density. Because a drop in color density can result in "cover over" by the next printed color, depletion is often accompanied by hue shift, compounding the problems caused by non-viable toner.
Several patents discuss liquid toner replenishment schemes for electrostatic or electrophotographic printing systems. U.S. Pat. No. 5,319,421 (West) and U.S. Pat. No. 4,222,497 (Lloyd et al.) teach replenishment of toner solids by measuring optical transmissivity of the liquid toner as it passes between two clear windows and relating this measurement to proper toner concentration. Toner concentrate is then added to the working strength toner based on these optical measurements.
U.S. Pat. No. 4,860,924 (Simms et al.) teaches the replenishment of toner based on measurement of working strength toner optical transmissivity and conductivity. Toner concentrate and charge control agent are added separately, and it is asserted that this method prevents the eventual depletion of the toner with repeated replenishment events. Also use of agitation to keep the toner concentrate from settling and the use of a motor-driven stirrer are disclosed.
U.S. Pat. No. 5,369,476 (Bowers et al.) teaches replenishment of toner based on measurement of the image quality. Also, the use of agitation to keep the toner concentrate from settling and the use of a recirculation pump are disclosed.
U.S. Pat. No. 5,155,001 (Landa et al.) teaches the use of a charge control agent that maintains proper concentration in the liquid toner by maintaining a solid-liquid phase equilibrium. A build up of charge agent with repeated replenishment events is avoided.
U.S. Pat. No. 5,442,427 (Day) teaches the use of agitation of toner concentrate which allows the use of concentrate with a lower concentration of charge control agent in the liquid toner formulation. The agitation keeps the concentrate suspended, and the decreased amount of charge control agent allows more replenishment events before the toner must be discarded because of high conductivity. Day and U.S. Pat. No. 5,404,210 (also Day) both disclose a means of recirculating toner through a purification apparatus that removes ionic contaminants from the toner. The purified toner is then added back to the system.
U.S. Pat. No. 5,623,715 (Clark) teaches (A) the use of continuous circulation of toner concentrate, in order to keep the particles from settling and allowing more precise addition of toner solids to the premix; (B) the use of a combination of a piston pump and check valves to precisely add toner concentrate; and (C) a calibration procedure for (B) whereby an analytical balance is used to weigh both imaged and non-imaged paper to determine the amount of toner solids applied to the paper, and calculate the concentrate replacement rate.
Several patents teach various mechanical means of removing depleted liquid toner from the development area in electrophotography. U.S. Pat. No. 3,808,025 and U.S. Pat. No. 3,913,524 (both Fukushima et al.) discuss passing a developed image and its adjacent layer of depleted toner through nip rolls in order to squeeze away the depleted toner. U.S. Pat. No. 4,623,241 (Buchan et al.) describes a slotted surface for removal of depleted toner from the development area. British Printed Specification 2179274-A (Spence-Bate) describes an apparatus that, combined with metering pumps, delivers a thin layer of liquid toner to the latent image in the exact amount used, so that excess toner need not be recirculated. In another embodiment, Spence-Bate also describes an alternative mode whereby excess toner can be drained away, and recirculated. Therefore, depleted toner that is not carried out with the image returns to the working strength toner reservoir, and it is not removed from the system.
These methods of eliminating depleted toner from the image surface and replacing it with fresh toner are essentially analogous to the toner applicator rollers used in multi-pass and single-pass electrostatic printers, such as those marketed by Raster Graphics of Sunnyvale, Calif., the ColorgrafX division of Xerox Corporation of San Jose, Calif., 3M Company of St. Paul, Minn., and N S Calcomp Corporation of Tokyo, Japan. Depleted toner is removed from the image surface, but is then mixed with the bulk of the working strength toner.
Several patents teach removal of liquid toner from electrographic printers in batchwise fashion. U.S. Pat. No. 5,396,316 (Smith) and U.S. Pat. No. 5,083,165 and U.S. Pat. No. 5,208,637 (both Landa) describe means of automating the step of discarding the depleted toner, by dispensing toner concentrate from a container or cartridge and removing excess depleted toner into the same container or cartridge.